The present invention relates generally to horizontal scan circuits for cathode ray tube (CRT) displays and particularly concerns a horizontal scan circuit capable of operation over a range of horizontal scan frequencies.
Computer driven video monitors are frequently operated in multiple resolution modes each often requiring a different horizontal scan frequency. In particular, the horizontal scan frequency at which the video monitor is operated is normally increased as the desired resolution is increased. For example, for a resolution of 700 lines a typical video monitor may be operated at a horizontal scan frequency of 15 KHz, with the horizontal scan frequency being increased to 32 KHz for a resolution of 1400 lines.
Horizontal scan oscillators capable of operation over a range of frequencies are well known in the art. Such oscillators normally drive a horizontal output stage for developing a horizontal output pulse signal which is applied to a horizontal yoke for horizontally deflecting an electron beam of the CRT and to a sweep transformer for developing the necessary high voltage for the CRT. It is well known that, in such systems, the amplitude of the horizontal output pulses is inversely related to the frequency of the horizontal scan oscillator. Thus, the amplitude of the horizontal output pulses will decrease as the horizontal oscillator frequency increases and visa versa. Such a decrease in horizontal output pulse amplitude with increasing horizontal scan frequency will result in a decreased horizontal deflection signal thereby reducing the horizontal size of the CRT display as well as a decreased high voltage signal.
In order to overcome these problems of reduced horizontal raster size and reduced high voltage when operating the horizontal deflection system of a video monitor over a range of scan frequencies, it has been proposed to switch between two essentially separate horizontal systems each tailored to provide proper operation of the monitor at a specific horizontal scan frequency. Switched dual horizontal deflection systems of this type require the use of a relatively complex and costly switching mechanism to switch between the two horizontal systems. In addition, since the switching mechanism, typically a multicontact relay, must switch a number of high voltage and current signals, transients are frequently generated which interfere with proper operation of the video monitor.
It is therefore a basic object of the present invention to provide an improved video display capable of operation over a range of horizontal scan frequencies.
It is a further object of the invention to provide a single, non-switched horizontal system for operating a video display over a range of horizontal scan frequencies while automatically maintaining substantially constant horizontal raster size and high voltage.
It is yet a further object of the invention to provide a video display capable of automatically changing an operating characteristic in response to operation at a predetermined horizontal scan frequency.